You can't silence someone with flags. The message can still be read as long as the moderator hasn't decided it was so rule breaking or destructive that the post needed to be removed. Posts that have been well liked by community members have been flagged to the point of being "hidden" (which you can easily get to show by just clicking the message stating it's hidden) by members that oppose the opinion, I've had this happen to myself multiple times. It doesn't happen on an organized level.
If flagging is mentioned in any of the discords I am in, it's usually just someone saying "I flagged that guy", sometimes stating a reason - usually reasons that are actually relevant to the forum rules!! But rarely if ever have I seen anyone go "guys please flag this person" or "lets flag him so his message is hidden". And if that happened, I think that person would be called out for attempting a conspiracy because quite frankly, the types of people in those discords is of as large a variety as they are in the forum! There's no way anyone would be able to create an organized attack in them lol, if anything things tend to be wilder in the discords because people use them more for emotional venting so that they can stay reasonable in the forums. AS THEY PROBABLY SHOULD, because if people laid their emotional beef out in the forums it would be HELL for moderators to deal with. If you want civilized conversation, you'll have to accept that people need somewhere else to take their emotions. And, on that note, you are not entitled to knowing every little detail of someone elses opinion. If there are certain feelings they would rather keep to themselves or somewhere else, they have every right to do so. That's not a conspiracy, that's just how privacy and social interaction works.
Multiple flags won't lead a mod to shut a thread down in and of itself. There have been occasions before where ratique flat out stated "stop flagging where a flag is not needed" and DIDN'T shut the thread down despite it overflowing with flags. The moderators make a judgement of their own and decide whether or not to ban members or shut a thread down based on the rules of the forum and based on how problematic they deem a thread has become.
I don't always agree with the moderators' decisions either. I've been flagged before too. But I will stand by the fact that there is no way you can genuinely manipulate the forums as a non mod using the flag system.
I've seen ACTUAL manipulation in a forum before. In that case, a neo nazi became friends with the most active moderator and thus managed to get away with continuously evading bans, kept coming back with sock puppet accounts to troll people he didn't like, and then convinced the moderator to ban the people that had enough and told him off for his troll behavior. This gradually led to the downfall of the entire forum, which eventually shut down. After which he tried to lure people to go to his forum instead.
I've seen manipulation in forums, this ain't it.